An IP Multimedia Subsystem, IMS, communications network is an example of a telecommunications system arranged for media exchange. Within the framework of IMS, a media session has a dialogue associated with it. The media descriptors related to a media session are exchanged by the dialogue between the respective end points of the dialogue. A dialogue is established within a Session Initiation Protocol, SIP, session. The SIP dialogue within a SIP session controls the media session taking place in the user plane of the IMS network.
In IMS there is a strict separation between control plane on one hand, and media plane or user plane on the other hand. SIP type signalling is conveyed through the control plane and controls the media session. On the other hand, media is transported through the user plane. Control plane signalling and user plane messages may be (but not necessarily are) transported through a different infrastructure in the IMS network, and thus may be subject to different routing paths between their respective end points.
FIG. 1 schematically illustrates the relation between a session initiation protocol session, and a session initiation protocol dialogue within the SIP session and, on the other hand, a media session. Between two end points User Agent-A, UA-A 3, and User Agent-B, UA-B 5, a SIP session 8 may convey multiple SIP dialogues between UA-A 3 and UA-B 5. In the example of FIG. 1, a session initiation protocol dialogue 9 is conveyed through the SIP session 8. The SIP dialogue 9 carries SIP signalling required for controlling a media session 10 also taking place between UA-A 3 and UA-B 5. This is schematically indicated by a dashed arrow 12 associating SIP dialogue 9 and media session 10.
The separation between the user plane and the control plane is also schematically illustrated in FIG. 2. In FIG. 2, UA-A 3 comprises a communication unit 16 and UA-B 5 comprises a communication unit 8. SIP signalling is conveyed through control plane 8, while the media session using a Real-time Transport Protocol, RTP, is conveyed through user plane 10. The IMS network is schematically indicated by a cloud 15 in FIG. 2.
As already mentioned above, a single SIP session may comprise multiple SIP dialogues relating to multiple media sessions in the user plane. An example of this can exist in situations wherein a SIP proxy server is involved, which applies the principle of “forking” in an attempt to find a recipient by searching multiple locations. Between the proxy server and each of the user agents approached by the proxy server, SIP sessions may exist conveying a SIP dialogue between the calling user agent and each of the respective potential recipients. Since the proxy server approaches multiple potential recipient user agents, SIP dialogues take place between the calling user agent and each of the potential recipient user agents approached by the proxy server. Between the proxy server and each of the recipient user agents, a single SIP session per recipient user agent conveys the respective SIP dialogue to that respective potential recipient user agent. Between the proxy and the calling user agent, a single SIP session is established which conveys all of the SIP dialogues between the calling user agent and each respective potential recipient user agent approached by the proxy server.
Accordingly, the SIP session between the calling user agent and the proxy server conveys a plurality of SIP dialogues, which are relayed by the proxy server to each of the respective potential recipient user agents. In addition, each of these SIP dialogues may control a media session between the calling user agent and the respective recipient user agent, the media session conveys data regarding a call to be established. Upon answering of the call by any of the respective recipient user agents, the SIP dialogue with that respective recipient user agent will become a confirmed dialogue. The SIP dialogues to the other respective recipient user agents will then seize, and with this also the media session between the calling user agent and each of the recipient user agents that have not answered the call will seize.
As explained above, a single SIP session may convey multiple SIP dialogues. In addition, it is noted that a single SIP dialogue may control multiple media sessions in the user plane. For example, a voice call may consist of a media session in the form of an RTP stream for the transfer of audio, and an associated Real-time Transport Control Protocol, RTCP, stream for controlling the audio data transfer. A video call may even consists of four respective media streams: an RTP stream for the audio, an RTCP control stream associated with the RTP audio stream, an RTP stream for video data, and an RTCP control stream associated with the video data, for example. All these media sessions, which are interrelated since they are all associated with one and the same call between two parties, may be controlled by a single SIP dialogue in a SIP session between the calling user agent and the recipient user agent.
SIP sessions and dialogues are used to set up and control communication between end points. Once a media session is established, it remains under control of the SIP dialogue under which control it was established. When a SIP dialogue ends, the related media session(s) will also end. When a User Agent, UA, terminates a SIP dialogue, that UA will also terminate the media session(s) controlled by that SIP dialogue. Likewise, when a UA receives an indication that a SIP dialogue is terminated or will be terminated, that UA will terminate the media session(s) controlled by that SIP dialogue.
As a result, while a media session is active, there must be a SIP dialogue corresponding to it. This SIP dialogue can be used to control the media session. If there is a need for a media session without the need to control this media session in any way any further by a corresponding early or confirmed dialogue, keeping the SIP dialogue active creates unnecessary overhead. This situation may, for example, occur when the media session does not require further (duration based) charging or quality control. Keeping the SIP dialogue active in that case implies that the SIP session has to have transited to the active state. Thus, the SIP session has to remain active even though there is no need for any further control of the media session that is established and controlled by the dialogue of this SIP session.
In view of the large and ever increasing amounts of (real-time) data conveyed by telecommunications networks nowadays, optimization in terms of data capacity at the networks itself and data handling capacity by servers becomes more and more important.